Attached is a report detailing a VFD failure in a critical warm room application. The VFD lacks a bypass, so I am exploring ways to enhance system reliability and failure recovery time. The AHU system operates 24/7 at approximately 56 Hz. After a failure analysis by Rockwell Automation suggesting overload or shock load heating, one risk reduction option is to replace the VFD with an across-the-line motor starter. Motor starters have fewer risk-failure components compared to VFDs, resulting in lower failure rates. While there is limited data comparing the failure rates or MTBF of motor starters and VFDs, my personal experience supports this claim. Your insights are appreciated. - Jeffro. (Attached report: Clips_from_my_report.doc, 28 KB).
I completely agree that in terms of component durability, a simple electromechanical starter is likely to be more reliable than a solid-state VFD. While I don't have specific data to support this claim, it is generally known that starters are more robust. Although there may be some efficiency trade-offs depending on the application, it seems that reliability is a top priority. Any modifications should be carefully evaluated. Two key considerations to keep in mind are: first, ensuring that the motor can safely handle the starting load on the power system when using direct-on-line starting. While a VFD may provide a gentler start for the motor, direct-on-line starting can put more strain on the motor, especially in cases of high load inertia or torque, low voltage, or high power system impedance. Secondly, it is important to assess whether the mechanical system can effectively handle extended periods of low heat load. Some systems, like centrifugal pumps or air conditioning compressors, may struggle to operate efficiently at low loads without the assistance of a VFD. For example, York 150 ton air conditioning units may experience excessive vibrations under low heat load conditions due to suction issues.
I am seeking reliable data on the performance of electrical components such as VFDs, motor starters, and transformers commonly used in AHU systems. While our system is designed to handle a motor running at 60Hz, an unexpected failure has prompted us to consider installing a bypass. However, management is pushing for alternative solutions. We typically have redundant systems in place for critical equipment, but bypass options for our AHUs were overlooked. Your mention of failure data reminded me of a previous incident with a Liebert UPS, where a circuit board failure led to a shutdown and bypass tripping. Despite attempts to obtain specific failure data from Liebert, they only provided generalized MTBF figures. I emphasized the importance of detailed failure history for predicting future reliability, but received no further assistance. It seems that manufacturers are hesitant to disclose true failure data, leaving maintenance groups to create their own benchmarks. Your insights are appreciated. Thank you.
Hello Jeffro, it looks like you've done a solid analysis on this tricky situation. I generally agree with your points about a motor starter having fewer failure-prone components. However, one thing to consider is the loss of efficiency and precision that comes with ditching the VFD. Although the failure rate might be higher, the energy savings and control the VFD offers often outperform an across-the-line starter in the long run. I'd recommend exploring hardware solutions like additional cooling or soft-start functionality to improve the VFD's reliability. Depending on your operation, the energy costs could outweigh the risks associated with VFD failure.
Hi Jeffro, thanks for sharing this. I agree, replacing the VFD with a motor starter could indeed reduce the risks associated with overload or shock load heating. But, do consider the trade-off in versatility as a motor starter will not provide the precise control of your AHU system like a VFD. By the way, another important factor to consider is your warm-room's temperature profile - sudden temperature changes could be detrimental since a motor starter lacks the ramping capability of a VFD. And, of course, consider the replacement and running costs. Having a properly configured protection in your VFD settings might be a cost-effective solution to think about as well.
Hi Jeffro, indeed, the option to switch to a motor starter might boost system reliability due to its elementary construction, but it may compromise your ability to finetune your precise operation at 56 Hz. An across-the-line motor starter's lack of variable speed control might present a problem there. But, if you can get by with a fixed speed, it's a good choice. Alternatively, you could invest in a higher quality VFD with an in-built bypass. This VFD bypass gives the added reliability similar to a starter during failure while still maintaining the variable frequency needed for precision control. Lastly, you might want to look into potential load anomalies in your AHU system, as repeated overloads can increase the failure rate of your VFD.
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Answer: - One potential strategy discussed is to replace the VFD with an across-the-line motor starter, which is suggested to have lower failure rates due to fewer risk-failure components compared to VFDs.
Answer: - The failure analysis suggested overload or shock load heating as potential causes of the VFD failure in the critical warm room application.
Answer: - Operating the AHU system at 56 Hz continuously may have contributed to the VFD failure. It's advisable to review the operating conditions and load requirements to ensure optimal performance and longevity of the equipment.
Answer: - Motor starters are suggested to have fewer risk-failure components compared to VFDs, potentially resulting in lower failure rates and improved failure recovery time. However, it's important to consider the specific requirements and control capabilities of the application before making a decision.
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